The present invention relates to a bearing assembly for supporting one end of a shaft for a position sensor in a thrust reverser for a turbofan jet engine.
FIG. 1 schematically illustrates a turbofan aircraft engine of known construction. A centrally located turbojet engine 1 delivers thrust by means of a primary jet flow indicated at F1. The turbojet engine 1 drives a fan 2 located at the front of the turbojet engine air intake such that a portion of the air flow generated by fan 2 exits through an annular secondary duct 3 as secondary flow F2 which produces additonal thrust. The secondary duct 3 is equipped with a thrust reverser 4 which is shown in its retracted position in the lower half of FIG. 1 and in its extended position in the upper half of FIG. 1. In its retracted position, it permits air to flow through the annular duct 3 as secondary flow F2. In its extended position, shutters 7 serve to redirect the secondary flow through baffles 6 in the direction of flow F'2. the housing of the secondary duct 3 comprises a fixed front portion 5a and a movable rear sleeve portion 5b which may translate axially by known control means. In the retracted position of the thrust reverser, sleeve 5b covers the baffles 6 and the shutters 7 conform to the inner wall of this sleeve so as not to impede the air flow therethrough.
In order to maintain safe flight conditions and to synchronize the various controls for the engine, the position of the thrust reverser must be reliably indicated. Typically, such indication means comprises a position sensor 8, shown in FIG. 2, which transmits the displacements and the position of movable sleeve 5b. In a known system, the position sensor 8 is located at one end of a shaft 9 which extends generally parallel to the axis of movement of sleeve 5b and is of sufficient length to accommodate the full range of movement of this sleeve. The sleeve 5b may traverse a distance of approximately 600 mm in some applications.
Shaft 9 is substantially cylindrical and has a helical cam 10 formed on its outer surface, the contour of the cam being a function of the servo control output of position sensor 8. The shaft 9 is rotatably supported on an upstream bearing 11 and a downstream bearing 12, both of which are fixedly attached to the fixed portion 5a of the secondary duct casing. A bushing 13 is attached to movable sleeve 5b and slides along the length of shaft 9. Bushing 13 has a slot therein to engage helical cam 10 such that, as bushing 13 and movable sleeve 5b translate axially, shaft 9 is rotated such that through its connection with position sensor 8, the position sensor produces an output signal indicative of the position of the movable sleeve 5b.
The upstream bearing 11 typically comprises a spherical joint 11a which is fixedly attached to shaft 9 and a spherical outer race 11b which is fixed to the upstream portion 5a of the secondary duct casing. The interengaging spherical surfaces of 11a and 11b allow the shaft 9 to rotate and will accommodate slight movement in directions transverse to the rotational axis. The center A of the spherical joint 11a is a stationary point of the shaft 9.
A known downstream bearing 12, only schematically illustrated in FIG. 2, is shown in more detail in FIG. 3. It is necessary to support the downstream end of shaft 9 in order to avoid any possibility of the rupture of this shaft due to the large overhang when the sleeve 5b is in its retracted position. The duct casing, fixed portion 5a and movable sleeve 5b, are subjected to high mechanical stresses during the engine operation, which may encompass cyclical deformation, which would substantially reduce the useful life of shaft 9 in the absence of a downstream bearing support. As a result of these stresses, shaft 9 undergoes both expansion and displacements in the x direction as well as orthogonal y and z directions.
In the prior downstream bearing support assembly illustrated in FIG. 3, the shaft 9 defines a recess 9a in its downstream end which cooperates with a shaft 14 via spherical joint 15 engaging a corresponding seat 18. A similar spherical joint comprising spherical ball 16 and spherical seat 19 attaches the opposite end of shaft 14 to a case 22. A third ball joint 17 engages spherical seat 23 between the ends of shaft 14. Nut 20 serves to retain the spherical seat 19 within bore 21 formed in case 22. Case 22 is fixed to stationary portion 5a by known means not shown. Spherical seat 23 is supported in plate 24 between shoes 25 and 26 which are slidably retained in an opening defined in case 22 by Belleville springs 27 and nut 28. Spherical seat 23 may slide axially within the bore 24a defined in plate 24 and the plate 24 can slide transversely between the shoes 25 and 26. The force compressing the assembly against the end of the bore in case 22 by the Belleville springs 27 may be adjusted by tightening or loosening nut 28. The assembly permits the downstream end of the sensor bar to move, while at the same time enabling the dampening of such movement to be adjusted.
This known design, however, has not fully obviated the problems associated with supporting the downstream end of shaft 9. Depending upon the displacements caused by the expansion and mechanical deformation of the shaft 9, the shaft 14 undergoes significant angular movement. Furthermore, the space to seat the damping elements, the plate 24, the shoes 25 and 26, and the springs 27 often renders the damping inadequate. This results in premature wear and quick maladjustment of the device causing a serious lack of reliability in the thrust reverser positioning system.